tadpoles of glass called prince rupert drops are both tough and fragile. bang them with a hammer, and they won't break. but snip their tails, and they shatter.
the curiosities were named for a 17th century prince who saw them at a bavarian glassworks and then impressed england's king charles 2nd with a demonstration. the drops are crude forms of tempered glass made by dripping molten glass into a bath of water or oil. this cools the exterior faster than the interior, creating surface compression that lends strength. stress lines are revealed (here) by cross-polarized light filters. a drop's tail is it's achilles' heel. there a mere scratch will set off an explosive chain reaction of fractures.
nowadays tempered glass - sometimes produced chemically rather than by quick cooling - can withstand pressure up to 22,000 pounds per square inch. it finds use in eyeglasses, oven cookware, basketball backboards, and car windows that shatter instead of breaking into lethal shards...
